The current definition of the JPEG (Joint Photographic Experts Group) standard does not allow for any form of adaptive quantization. Adaptive quantization can significantly improve the image quality achieved at a given bit rate.
Adaptive quantization is not new. For example, one scheme for adaptive quantization is disclosed in U.S. Pat. No. 4,922,273 to Yonekawa. Yonekawa teaches a technique for automatically adjusting quantization based on activity measures derived from the Discrete Cosine Transform (DCT) coefficients. In addition, in an MPEG scheme (Moving Picture Experts Group, ISO-IEC/JTC1/SC2/WG11), the DC coefficient is fixed while a single scaling factor is used to adjust the quantization levels for all 64 coefficients produced by the 8.times.8 DCT. Another possible method for adapting the quantization is to individually change the value of each of the 64 quantization values.
There exist alternative methods to achieve adapting of the quantization within the JPEG standard. However, none of these alternatives can be used in a simple sequential mode, as all require full DCT or image buffering in some form. One possible alternative for transmitting adaptive quantization information is the use of one of the JPEG Application Marker Segments. The disadvantages of this alternative are that these Marker Segments are currently undefined and individual implementers are not permitted to define them. The adaptive quantization always has to be transmitted separately from the actual coded image data, which would not be appropriate when using Marker Segments. Finally, Marker Segment require a non-standard algorithm for the coding of the quantization values.
A simple form of adaptive quantization can be achieved with the JPEG successive approximation progressive mode. Basically, at some point in the encoding process, the coefficients in regions which need only coarse quantization are not improved as further successive approximation scans are coded. Effectively, the quantization is coarser by a power of 2 in those regions if a successive approximation progressive mode technique is used. The disadvantages of this approach are the need for a full progressive implementation, the relatively coarse scaling of quantization values, the development of a "fat zero" for very coarse quantization.
A similar form of adaptive quantization can be realized within the JPEG sequential mode. In the encoder, low magnitude bits are cleared in regions where coarser quantization is desired. This has the same effect as not updating some regions in the progressive mode, therefore, except for operating sequentially, suffers from the same objections detailed above.
Still another alternative form for adaptive quantization could use the JPEG hierarchical mode. In this case, refinement of the quantization would require a second frame in which selective parts of the image are improved.